The invention relates to stimulation of wells penetrating subterranean formations. More particularly it relates to acid fracturing; most particularly it relates to methods of differentially etching the fracture faces of sandstone formations so that etching provides a conductive path from the fracture tip to the wellbore.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
There exist several stimulation treatments for increasing production from hydrocarbon reservoirs, such as fracture stimulation. Proppant fracturing treatments consist of injecting fluid into a well at a certain rate and under a certain pressure for fracturing the reservoir and fixing the fracture with a propping agent that prevents the fracture from closing. This method is used for both carbonate and sandstone formations.
Acid fracturing is an alternative to proppant fracturing and is commonly used in the treatment of carbonate formations with the same objectives of creating long, open, conductive channels from the wellbore, extending deep into the formation. The difference between the two fracturing methods is in how the fracture conductivity is created and maintained. Fracture acidizing does not utilize proppant to hold the fracture open, but relies on the non-uniform etching of fracture faces with acid, resulting in the formation of conductive channels. Such acid fracturing has been reserved exclusively to carbonate formations.
The treatment of carbonate formations with acid works well because the favorable kinetics of carbonate dissolution by the acid. A variety of different acids may be used in treating carbonate formations. The etching of carbonate surfaces of the formation appears to be non-uniform due to inhomogeneties in the formation composition that lead to uneven reaction with the rock. This may result in disparities in the opposing fracture faces that do not match up when the fracture pressure is released so that “pillars” are formed that support the fracture wall after closure and provides conductive channels or flow paths to facilitate production of fluids from the formation to the wellbore.
In contrast to carbonate formations, sandstone formations are only susceptible to dissolution by hydrofluoric acid. The reaction-rate-limiting kinetics of sandstone dissolution by hydrogen fluoride results in uniform etching of the sandstone surfaces so that no pillars or channels of the formation are formed after the fracture has closed.
Accordingly, a need exists for a method for using acid fracturing techniques in sandstone formations to form conductive fractures.